1. Field of the Invention
The present invention relates to the field of acousto-optic correlators.
2. Prior Art
The basic space integrating (SI) optical acousto-optic correlator architecture and concept is well understood. [R. M. Montgomery, "Acousto-optical Signal Processing System", U.S. Pat. No. 3,634,749 (January 1982)] This basic system is limited in the signal duration and range delay that can be processed. The concept of using repeated or cyclic active reference signals in this architecture is likewise well known [D. Casasent, "Frequency-Multiplexed Acousto-Optic Architectures and Applications", Applied Optics, Vol. 24, pp 856-858 (March 1985)]. This cyclic reference signal technique is attractive because it allows an infinite range delay search (essential for synchronization applications), but it significantly limits the maximum signal duration that can be handled to T.sub.A /2 (where T.sub.A is the aperture time of the system, i.e. the aperture time of the AO cells used). Prior techniques advanced to overcome these problems have used the well-known time-integrating (TI) AO correlator architecture [R. A. Sprague and C. L. Koliopoulos, "Time-Integrating Acousto-Optic Correlator", Applied Optics, Vol. 15, pp 89-92 (1976)] with single and multi-channel AO cells, multiple input point modulators and frequency-multiplexing [D. Casasent, supra; D. Casasent, "General Time, Space and Frequency-Multiplexed Acoustic Correlator", Applied Optics, Vol. 24, pp 2884-2888 (15 Sept. 1985)]. These architectures require the detection of separate portions of the full large duration signal correlation and the subsequent proper delay and summation of these "mini-correlations". To allow this, complex correlations are necessary. No one has yet detailed how to achieve these complex correlations on acousto-optic processors. In any time integrating (TI) architecture, an excessive number of detectors is required to achieve this (with no processing gain and noise performance loss). The present invention method described herein can be used to accomplish this in a TI system, though such a system is not detailed further herein because the output detector requirements (the number of detectors) is excessive. Thus the present disclosure is directed only to an SI AO correlator. In correlations of coded signals with phase modulation, one must be able to achieve either coherent or noncoherent detection. This issue has not been noted in prior acousto-optic detection correlators. The present invention enables one to achieve both coherent and noncoherent detection.